Understanding GIS. David Smith

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      5)Close the table, histogram, and Chart Properties pane when you are finished looking at them.

       Symbolize census tracts by population density

      Symbolizing a layer by an attribute, also called thematic mapping, allows you to see how values are spatially distributed.

      1)With the tracts layer selected in the Contents pane, open the Symbology pane by clicking Symbology on the Appearance ribbon.

      By default, all features in a layer have a single symbol. That’s why all your census tracts are purple, or whatever color they happen to be.

      2)In the Symbology drop-down list, click Graduated Colors. The map updates automatically on the basis of some default values.

      3)In the Field drop-down list, click POPDENS_CY. On the map, the tracts are symbolized by population density.

      4)Change the Color scheme to Yellow to Red.

      You can view the names of the schemes by selecting the Show names check box at the bottom of the list. Additional color schemes are available by selecting the Show all check box.

      A lot is going on here if you expand the Symbol chart. (The same information is also available in the Contents pane.) The values for the POPDENS_CY attribute, which range from 0 to 96,824.5, are divided into five classes. The starting and ending values for each class are calculated by a “natural breaks” algorithm that separates clumps in the data. That’s why the range of values is different from class to class and why classes break at seemingly arbitrary numbers. Each class is associated with a symbol in a color ramp and is displayed on the map.

       Change the classification

      Quantitative symbology is flexible, and you can present data in many ways. Because all you want right now is a general sense of viable areas for your project, and because you’re going to look at a couple of variables together, you should keep your presentation simple.

      1)Change the Classes drop-down box to 3.

      2)Set the classification method to Quantile.

      Using three classes, you’ll easily be able to see high, medium, and low values. The quantile method guarantees that an equal number of tracts will fall into each class. It should be noted that there are no inherently good or bad ways to classify data—different classifications may be more or less appropriate to the purpose of your map and the background knowledge of your audience.

      3)Change the first upper value to 8000 by double-clicking the label, typing the value into the Upper value cell, and pressing Enter. The second class break point is selected and editable.

      4)Change the second upper value to 16000.

      Notice that the classification method has been reset to Manual Interval because you’ve changed the class breaks. The histogram is updated, too. You no longer have a pure quantile classification, but having your classes break at round numbers makes intuitive sense.

      The outlines of the polygons are currently overwhelming the map in some of the areas with the highest densities, making it difficult to interpret the classifications.

      5)Click the More button on the Classes tab and select Format all symbols. Any changes made here will apply to all the classifications.

      6)If necessary, change to the Properties tab (near the top of the pane). Change the Outline color to No Color.

      7)Click Apply at the bottom of the pane, and then click the back arrow at the top of the Symbology pane to return to the classification settings.

      You’re removing the outlines because you don’t need to see the tract boundaries on the map. For now, you’re interested in them as areas, not specifically as tracts.

      8)In the Label column, double-click on the first label (< 8000) to make it editable. Type Low, and press Enter.

       If you start typing in the wrong box (in this case, Upper value), change the classification method to Natural breaks to reset the upper value, and then begin typing in the correct box, Label.

      9)Replace the second label with Medium. Press Enter.

      10)Change the third label to High. Click outside the edit box to commit the edit.

      11)Change the transparency of the tracts layer to 50% (from the Appearance ribbon).

      12)In the Contents pane, drag the tracts layer under the Parkland layer.

      On the map, you can now see where population is concentrated along the river, and you can see it in relation to existing parks.

       Measure distance from the river to parks

      Making a few measurements will improve your ability to estimate distance on the map and give you a better intuitive sense of how close to the river the new park should be.

      1)Select the Dodger Stadium bookmark (if necessary, turn off the tracts layer).

      2)Pan the map so that a number of parks are in the view. Feel free to zoom in or out.

      3)On the Map tab, in the Inquiry group, click the Measure > Measure Distance button to open the Measure Options.

      4)Click the Options drop-down arrow. Set the Distance units to Miles and turn off Feet.

      On the map, the pointer changes to a ruler with inscribed crosshairs.

      5)Point to a park, such as Cypress Park (northeast of Dodger Stadium on the east side of the river).

      6)Click to start a measurement.

      7)Point (you don’t have to drag the pointer) to the river. The measurement result is displayed along the line and in the Measure dialog box.

      8)Double-click to end the measurement.

      9)Click on another park and measure its distance to the river.

      The new result replaces the previous one in the Measure dialog box.

      10)Measure СКАЧАТЬ